This invention pertains to an apparatus for detecting load weight on a conveyor belt, and more particularly, to such an apparatus which rejects all forces other than that created by the load weight on the conveyor belt.
A conventional conveyor belt system includes a plurality of rollers mounted on axles. An endless belt is looped around the rollers. One of the rollers is driven so that the belt is continuously moving over the rollers.
In the prior art, there are several devices and methods for detecting the weight of a load on a conveyor belt. Many of the prior art devices utilize a strain gauge attached to a portion of the roller support structure which deforms proportionately to the load on the belt. The gauge's resistance varies as it deforms, and by applying a voltage across the gauge, an electrical signal is generated which is proportionate to the weight of the load on the belt at a given instant in time.
Several problems exist with prior art devices which cause inaccurate measurements of a load on a belt. In some devices, it is necessary that the load be distributed evenly across the width of a belt in order for an accurate reading to be obtained. Also, in the prior art devices, it is possible for the shafts upon which the conveyor rolls are mounted to become heated (due to friction) and to expand, producing forces along the axles on which the conveyor rollers are mounted. These forces are ultimately detected by strain gauges, causing an inaccurate indication of load on the belt. At least one system carries the entire weight of a roller and axle on a gimbal which rests on a load cell. This system minimizes forces generated along the axles of the rollers. However, since the load cell always bears the entire weight of the roller and support structure for the roller, the output signal has a very poor signal-to-noise ratio, and it is therefore difficult to obtain an accurate load measurement. Another drawback is that such an arrangement is sensitive to torque, and thus can easily produce error indications.
A general object of the present invention is to provide a practical, easily installed, apparatus for detecting load weight on a conveyor belt, which apparatus eliminates the above-stated problems in the prior art.
According to a preferred embodiment of the invention, the proposed apparatus is for use with a conveyor belt supported by an elongated roller which is carried on an elongated axle having ends projecting from the roller. The apparatus includes a pair of axle supports which are substantially planar plates whose planes are oriented substantially normal to the rotational axis of the roller. Each plate has, for example, a displacement detector (deformation detector) connected thereto. The displacement detector deforms in proportion to the load on the plate to which it is attached. When the detector deforms, its electrical resistance varies proportionate to the amount of deformation. Each detector comprises two pairs of strain guages, or the like, each pair including one gauge which compresses while the other tenses, both in response to a vertical force on the support. Each support has such gauge pairs mounted on its opposing planar surfaces.
All of such strain gauges included in the apparatus are connected to form one-half of a bridge circuit, the other half being formed by a potentiometer. A meter is used across the bridge circuit. With this configuration, the only forces which generate signals through such a meter are vertical forces in the axles supports, representing only the load weight on the conveyor belt.
These and other objects and advantages attained by the invention will become more fully apparent as the description which follows is read in conjunction with the accompanying drawings.